2. Field of the Invention
The present invention is directed generally to arc torches and more particularly to arc torches of the transfer type.
2. Description of the Prior Art
Plasma arc torches are well known in the art as seen, for example, by U.S. Pat. No. 4,144,444 to Dementiev et al., issued Mar. 13, 1979. FIG. 1 illustrates how the arc torch of the aforementioned patent functions. In FIG. 1, a torch 10 includes a rod-type electrode 12, a hollow, cylindrical electrode 14 and a diaphragm 16. The electrode 14 is larger in diameter than the diaphragm 16. A solenoid 18 is disposed about the electrode 14. An arc is initially struck across the gap between the electrode 12 and the diaphragm 16. An inlet 24 serves to inject a carrier gas which causes the foot of the arc, which is attached to the diaphragm, to move downwardly along the diaphragm until it transfers to the electrode 14. Thus, the arc is made up of a longitudinally-blown portion 20 disposed in the diaphragm 16 and a cross-blown portion 22 disposed in the electrode 14. The cross-blown arc portion 22 is caused to rotate in a magnetic field generated by the solenoid 18. Inlets 25 allow a feedstock gas to be injected into the cylindrical electrode 14 where it is heated by the rotating portion of the cross-blown arc 22. An outlet 26 serves to discharge the heated gas.
Because of the ability of an arc torch to heat a gas, fluid or other matter to extremely high temperatures, arc torches have found uses in a wide variety of applications. Arc heaters have been used for melting metal chips as disclosed in U.S. Pat. No. 4,214,736 to Wolf et al., for processing magnetite spheres as disclosed in U.S. Pat. No. 4,071,588 to Fey et al., and for reducing complex metallic ores as disclosed in U.S. Pat. No. 3,997,333 to Fey. Arc torches have also been used for firing an iron blast furnace as disclosed in U.S. Pat. No. 4,247,732 to Fey and for igniting pulverized coal as the coal is fed in an air stream through the coal burners of a conventional coal fired steam boiler as disclosed in U.S. Pat. No. 4,089,628 to Blackburn.
Plasma arc torches have also been put to such varied uses as welding, as disclosed in U.S. Pat. Nos. 3,949,188 to Tateno, 4,250,373 to Tanida, and 3,928,745 to Demars et al., surfacing processes as disclosed in U.S. Pat. Nos. 4,140,892 to Muller and 4,125,754 to Wasserman et al., and cleaning as disclosed in U.S. Pat. No. 4,555,612 to Collins, et al.
Despite the wide variety of applications for plasma torches, such torches may be classified into two general types: transferred and non-transferred torches. In the non-transferred torch, the arc is completely contained within the torch and extends from an upstream electrode to a downstream electrode. In applications where it is desirable to produce a stream of super-heated gas, the non-transferred type of torch is often used. For example, see aforementioned U.S. Pat. No. 4,214,736 used in an arc heater melting system, U.S. Pat. No. 3,997,33 used in a process for the reduction of complex metallic ores, U.S. Pat. No. 4,247,732 used in a method for electrically firing an iron blast furnace, and U.S. Pat. No. 4,089,628 used for igniting a stream of pulverized coal.
In the transferred type of arc torch, the foot of the arc leaves one of the electrodes to attach to another surface, typically the workpiece. In this regard, arc torches used for welding such as U.S. Pat. Nos. 3,949,188 and 3,928,745 and arc torches used for surfacing such as U.S. Pat. No. 4,125,754 are examples of transfer type arc torches.
In addition to classifying torches on the basis of being a transfer or a non-transfer type of torch, torches may be classified according to the configuration of the electrodes. In one configuration, two cylindrical electrodes are provided which are oriented colinearly. In this orientation, a small gap is provided between adjacent ends of the cylindrical electrodes in which the arc is struck. Thereafter, a carrier gas is used to blow the arc out of this gap. In this regard, see U.S. Pat. No. 4,559,439.
In a second configuration, one of the electrodes is shaped like a rod and is positioned within a second electrode which is cylindrically shaped. The arc is struck in the gap between the rod shaped electrode and cylindrically shaped electrode. A carrier gas is used to blow the arc to the tip of the rod shaped electrode at which time the foot of the arc may be transferred from the cylindrical electrode to a workpiece. In this regard, see U.S. Pat. No. 3,928,745.
Although arc torches have been used in a wide variety of applications, the need exists for a torch configuration which provides for extended electrode life. There is also a need for an arc torch designed so that the electrodes may be easily replaced when they do become worn or eroded. It is also desirable to design an arc torch which is easy and inexpensive to manufacture. Overriding all of these considerations is the need to have an arc torch which produces an arc which is stable and controllable, and a design which is easily adaptable for high current and high power applications.